1. Field of the Invention
The present invention relates generally to electrical noise filtering, and more particularly, to filtering of high-frequency ripple noise from electrical circuits.
2. Description of the Related Art
When an alternating current (AC) passes through a conductor, depending on the alternating frequency, the current tends to crowd itself near the surface of the conductor. This phenomenon is called the xe2x80x9cskin effectxe2x80x9d and is known in the art.
FIG. 1 diagrammatically depicts the skin effect in which a cross-sectional area A of a conductor 2 carrying an AC current i in a direction 4 which is perpendicular to and out of the figure. Depending on the frequency of the AC current i, the current i tends to propagate near the surface S of the conductor 2. The higher the frequency, the more the current i travels proximally to the surface S. Skin effect arises from the fact that the current i at different points in the cross-sectional area A does not encounter equal inductance. The current i confronts higher inductance, specifically self-inductance, near the center C of the cross-sectional area A. At the same time, there is much less self-inductance experienced by the current i near the skin surface S of the conductor 2. As a consequence, an uneven current distribution results. The current density near the skin surface S is much higher than the corresponding current density near the center of the area A. With the majority of the current crowds near the conductor skin surface S which has a limited cross-sectional area in comparison to the total area A, the uneven current distribution in effect increases the overall resistance of the conductor 2.
Two main factors contribute to the skin effect, they are, namely, the cross-sectional area of the conductor and the frequency of the current passing through the conductor. Referring back to FIG. 1, when the current i passes through a large cross-sectional area A, even at low frequency such as 60 Hz (Hertz), skin effect can be eminent. Alternatively, effective resistance also substantially increases when the AC current i alternates at high frequency, even with the cross-sectional area A relatively small in dimension. However, most detrimental of all is when the current i passes through the conductor 2 with a large cross-sectional area A and alternates at high frequency.
In reality, the combination of the aforementioned factors occurs in the design of power supplies and related circuits. For example, shown in FIG. 2 is a side elevational view, somewhat schematically, of a section of a pair of power busbars 6 and 8 driving a load 9. The busbars 6 and 8 are in turn driven by a power supply 5. As is known in the art, to supply a high current output with a manageable physical size, the power supply 5 needs to be built as a switch-mode power supply. Modern day power supplies are mostly designed to be miniaturized in size. To accomplish this end, switching frequencies of the power supplies are driven at very high ranges. The rapid switching power supply 5 is a source of high frequency noise which, if not properly controlled, is detrimental to the operation of the load 9. The noise problem associated with the power supply 5 will be explained further below.
FIG. 3 is a cross-sectional view taken along the line 3xe2x80x943 of FIG. 2. The busbars 6 and 8 can be found in a high-current output power supply used to drive Internet routers, for example. The busbars 6 and 8 respectively carry currents 10 and 12 as shown in FIGS. 2 and 3. The directions of current flow for currents 10 and 12 are opposite to each other in this case. Depending on the load 9, the currents 10 and 12 may assume high values. As such, the busbars 6 and 8 are normally designed with large cross-sectional areas. Each busbar 6 or 8 is intended to carry a direct current (DC). However, superimposed on the DC component of each current 10 or 12 is normally high-frequency noise. FIG. 4 graphically shows a typical current characteristic of the busbar 6 or 8. As shown in FIG. 4, there is a DC component signified by the reference numeral 14, and a noise component labeled 16. The noise 16 comes from a variety of sources with a wide spectrum of frequency ranges. As mentioned above, modern day fast switching power supplies substantially aggravate the noise problem. For example, a major portion of the noise 16 may come from switching rectifiers and MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) which are key components of the power supply 5.
The presence of the noise 16 is undesirable in several respects. Chief among all is the impairment of operational reliability of the load 9. The noise 16 can cause false triggering of logical circuits, as well as erroneous electrical level processing of analog circuits in the load 9. The noise 16, if unmanaged, can render the load 9 totally malfunctional.
Not to be overlooked is the fact that the noise 16 is high-frequency in nature and the skin effect normally takes into effect. For reasons as stated above, the noise 16 tends to crowd itself near the skin surface of the busbars 6 and 8 when propagating through the busbars 6 and 8, as diagrammatically shown in FIG. 3. The noise currents passing through the limited cross-sectional areas near the busbar skins generate unwanted heat which further aggrevates the reliability of the components placed adjacent to the busbars 6 and 8. Quite often, in high-frequency applications, the heat generated can be substantial.
For reasons as stated above, the noise 16 is normally filtered away before reaching any load. Heretofore, filtering of noise on the busbars 6 and 8 has mostly been accomplished by placing noise filtering capacitors 18 between the busbars 6 and 8, irrespective of the distance between the busbars 6 and 8. As a consequence, only a portion of the noise is filtered. That is, noise is filtered only in the areas at or near the capacitors 18. The majority of noise 16 away from the capacitors, such as the noise at the three skin surfaces 6A, 6B and 6C of the busbar 6 remain intact and may not be affected at all. Similarly, noise 16 clustering at the other three surface skins 8A, 8B and 8C away from the capacitors 18 on the busbar 8 may also escape filtering and goes directly to the load 9.
High frequency switching mode power supplies are commonly used to power electronic circuits. There has been a long-felt and increasing need to effectively and efficiently suppress the unwanted noise in practical applications.
It is accordingly the object of the invention to provide a circuit scheme which efficiently filters away unwanted noise, thereby improving operational reliability and curtails wasteful heat generation. The objective of effective noise filtering without resorting to elaborate and expensive implementation is also sought.
The noise filtering assembly of the invention includes first and second busbars. The first busbar is proximally spaced from the second busbar at a fixed distance sufficient to induce proximity effect. As arranged, when current passes through the busbars, the high frequency noise having higher order harmonics adhere to the surfaces spaced by the fixed distance. Noise can be removed efficiently by disposing low equivalent resistance noise filters between the surfaces where the noise harmonics concentrate.
In other embodiments, more than two busbars are proximally disposed together so as to enhance the proximity effect. Noise filters are also disposed on the busbar surfaces where the noise concentrates.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which like reference numerals refer to like parts.